Different Kinds of Alcohol Fuels

Thursday, June 9, 2011

Alcohol has been used as a fuel for a very long time. Four alcohols are of interest as fuels because they can be synthesized chemically or biologically, and they have characteristics which allow them to be used in current engines:
Methanol
Ethanol
Propanol
Butanol
One advantage shared by all four alcohols is their high octane rating. This tends to increase fuel efficiency and partially offsets the lower energy density of alcohol fuels (as compared to gasoline and diesel fuels), thus resulting in better "fuel economy" in terms of miles per gallon than it would seem from a simple comparison of energy density.

Butanol has an advantage as a fuel because its energy density is closer to gasoline than the simpler alcohols (while still retaining over 25% higher octane rating); however, biobutanol is currently more difficult to produce than ethanol or methanol.

Most methanol is produced from natural gas, although it can be produced from biomass using very similar chemical processes. Ethanol is commonly produced from biological material though a fermentation processes. When obtained from biological materials and/or biological processes, alcohols are known as bioalcohols (e.g. bioethanol).

Butanol has a high flashpoint of 35 °C, which is a benefit for fire safety, but may be a difficulty for starting engines in cold weather. The concept of flash point is, however, not directly applicable to engines as the compression of the air in the cylinder means that the temperature is several hundred degrees Celsius before ignition takes place.

The fermentation processes to produce propanol and butanol from cellulose are fairly tricky to execute, and the Weizmann organism (Clostridium acetobutylicum) currently used to perform these conversions produces an extremely unpleasant smell, and this must be taken into consideration when designing and locating a fermentation plant. This organism also dies when the butanol content of whatever it is fermenting rises to 7%. For comparison, yeast dies when the ethanol content of its feedstock hits 14%. Specialized strains can tolerate even greater ethanol concentrations — so-called turbo yeast can withstand up to 16% ethanol.

But if ordinary Saccharomyces yeast can be modified to improve its ethanol resistance, scientists may yet produce a strain of the Weizmann organism with a butanol resistance higher than the natural boundary of 7%. This would be useful because butanol has a higher energy density than ethanol, and because waste fiber left over from sugar crops used to make ethanol could be made into butanol, raising the alcohol yield of fuel crops without a need for more crops to be planted.

Despite these drawbacks, DuPont and British Petroleum have recently announced that they are jointly to build a small scale butanol fuel demonstration plant.

Energy Environment International developed a method for producing butanol from biomass, which involves the use of two separate micro-organisms in sequence to minimize production of acetone and ethanol byproducts.

Butanol may be used as a fuel in an internal combustion engine. Because its longer hydrocarbon chain causes it to be fairly non-polar, it is more similar to gasoline than it is to ethanol. Butanol has been demonstrated to work in vehicles designed for use with gasoline without modification. It can be produced from biomass as well as from fossil fuels.

This article was adapted from the article, Alcohol Fuel.

Read more: Interesting Facts About Alcohol Fuels.

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